1. Field of the Invention
This invention relates to a heat transfer system for controlling the temperature of a battery. More specifically, this invention relates to apparatus and methods useful in dissipating heat generated by operation of a battery used in a spacecraft or an electrical vehicle through a terminal of the battery.
2. Brief Description of the Related Art
A typical battery comprises a plurality of cells in a stack. Each cell has one or more electrodes and a pair of terminals. Interconnectors connect one cell terminal to another cell terminal. An outer casing is disposed around the cells. The outer casing is typically electrically insulated from the terminals. An electrically insulating layer is located between the inner surface of the outer casing and the cells.
Waste heat generated in the electrodes of the battery cell must be conducted away from the cell to keep a battery at a temperature suitable for operation. Spacecraft batteries typically accomplish this by conducting heat on a thermal conduction path from the electrodes to the outer casing of the battery and then through a thermal conduction path (e.g., a heat pipe) to a heat sink (e.g., a radiator). The thermal conduction path from the cell stack to the outer casing of the battery has a high internal thermal resistance because between the edge of the cell stack and the outer casing there is a gap or insulator for electrical insulation which has a high thermal resistance compared to metal-to-metal contact.
Dissipation of generated heat via the aforementioned route is convenient because each of the cell terminals, electrodes, and cell interconnectors are necessary electrical components of a battery regardless of how little heat must be dissipated. Conventional battery cells for high current applications, such as spacecraft or electric vehicles, have heavy electrical conductors, called interconnectors, connecting one cell to another. In addition, each cell has a heavy thermal conductor connecting the cell case to the radiator. Typically, the only element necessary for the sole purpose of controlling the temperature of the battery is the aforementioned conductor which, when used in a spacecraft, weighs about sixty pounds.
Conductive heat transfer via the aforementioned route is suitable in situations where the weight of the battery is not a concern. However, where the weight of the battery is a concern, the typical heat conduction paths impose restrictions on the use of the battery in certain applications. Such a weight restriction exists when a battery is used in a spacecraft where each unit of weight corresponds to an incremental unit of cost. For example, the cost to transport one pound of hardware into space is about $20,000 (1997 dollars). Thus, the benefits of any weight reduction would immediately result in a substantial cost savings.
In some conventional battery cells, heat produced in the electrodes must pass through an insulating gas to reach the outer casing of the cell. The heat must then cross the cell outer casing, which is typically a poor thermal conductor, and pass through a layer of insulation outside of the outer casing prior to reaching the thermal conductor. Thus, there is a need for battery cells with shorter, more efficient thermal conduction paths.
Changes in cell pressure, which may occur in space applications, cause the outer casing to move which may cause debonding of the outer case from the thermal conductor. The debonding greatly reduces the thermal contact, thereby decreasing the efficiency of heat transfer from the outer casing to the thermal conductor.
It would be desirable to provide an apparatus that can remove heat generated in a battery cell, wherein the apparatus remains generally unaffected by variations in cell pressure. Furthermore, it would be desirable to eliminate the necessity of a heavy thermal conductor external to the battery cell, yet provide a suitable means for controlling and dissipating the heat generated within the battery cell. Still further, it would be desirable to provide a weight-conservative apparatus and methods for controlling and dissipating the heat generated by operation of a battery cell.